The present invention relates to treatment fluids useful in subterranean applications and more particularly, to crosslinkable-polymer compositions that comprise a chitosan-based compound and a phenol source, and associated methods.
In certain subterranean formations, it may be desirable to mitigate the flow of fluids through a portion of the subterranean formation that is penetrated by a well. In some instances, it may be desirable to control the flow of fluids introduced into the well so that the flow of the fluid into high-permeability portions of the formation is mitigated. For example, in an injection well, it may be desirable to seal off high-permeability portions of a subterranean formation that otherwise would accept larger portions of an injected treatment fluid. By sealing off the high-permeability portions of the subterranean formation, the injected treatment fluid thus may penetrate less permeable portions of the subterranean formation more effectively. As used herein, the term “treatment,” or “treating,” refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment,” or “treating,” does not imply any particular action by the fluid or any particular component thereof.
In other instances, it may be desirable to mitigate the production of undesired fluids (e.g., water) from the well. The production of water with oil and gas from wells constitutes a major problem and expense in the production of oil and gas. While oil and gas wells are usually completed in hydrocarbon-producing formations, a water-bearing zone occasionally may be adjacent to the hydrocarbon-producing formation. In some instances, the higher mobility of the water may allow it to flow into the hydrocarbon-producing formation by way of, among other things, natural fractures and high-permeability streaks. In some circumstances, the ratio of produced water to hydrocarbons may, over time, become sufficiently high that the cost of producing, separating, and disposing of the water may represent a significant economic loss.
One technique that may mitigate the flow of fluids through a portion of a subterranean formation has been to place crosslinkable-polymer compositions in a well bore so as to cause them to enter the portion of the subterranean formation such that they may crosslink therein. The crosslinking of these compositions tends to produce crosslinked gels, which may eliminate, or at least reduce, the flow of water or other undesirable fluids through the natural fractures and high-permeability streaks in the formations. One particular crosslinkable-polymer composition involves the use of chitosan to crosslink a water-soluble polymer, such as an acrylamide-based polymer.
Chitosan is a special member of a general class of polysaccharides or carbohydrate polymers in the sense that it is composed of aminoglucose units instead of glucose units. Chitosan is a beta-(1-4)-polysaccharide of D-glucosamine and is structurally similar to cellulose, except that the C-2 hydroxyl group in cellulose is substituted with a primary amine group in chitosan. All carbohydrates, including chitosan, thermally degrade at different rates when heated to temperatures about 300° F. Chitosan usually occurs in nature in small amounts and typically is biodegradable. Chitosan-degrading enzymes, namely chitinases, chitosanases, and lysozymes that degrade chitin-derived materials occur in bacteria, fungi, algae, mammals, birds, fish, etc. Chitosan is a partially or fully deacetylated form of chitin. Chitin is typically a naturally occurring polysaccharide. Structurally, chitin is a polysaccharide consisting of beta-(1-4)-2-acetamido-2-deoxy-D-glucose units, some of which are deacetylated. Chitin is not one polymer with a fixed stoichiometry, but a class of polymers of N-acetylglucosamine with different crystal structures and degrees of deacetylation and with fairly large variability from species to species.
The time required for a crosslinkable-polymer composition to form the desired crosslinked gel can vary widely. This length of time, sometimes referred to as “gelation time,” varies, depending on a number of factors, including the type of crosslinking agent used, the type of polymer used, the type of aqueous fluid used, concentrations of components used, the pH, the temperature, and a variety of other factors. Delaying the gelation of a crosslinkable-polymer composition may be desirable to allow, among other things, pumping of the composition to its desired location. The desired gelation time varies depending on the specific application. For instance, for wells of considerable depth or increased temperature, a longer gelation time may be required to deliver the crosslinkable-polymer composition to its desired destination before the composition forms the crosslinked gel.
In subterranean formations, a wide range of temperatures may be encountered that may present challenges to the use of crosslinkable-polymer compositions therein. For example, if the temperature of the subterranean formation is sufficiently high, the crosslinkable-polymer composition may gel prematurely. To counteract this possibility, oftentimes, the crosslinkable-polymer composition may be designed such that its gelation time is delayed or retarded. That is, the thickening and gelation characteristics of the crosslinkable-polymer composition may be altered such that the time it takes the crosslinkable-polymer composition to form a crosslinked gel is delayed for an amount of time sufficient to permit the crosslinkable-polymer composition to be pumped to its desired destination.
Oil well completion fluids such as drilling fluids, spacer fluids and flushes containing natural polymers, such as gums, starch and cellulose derivatives, for the purpose of fluid loss control or for particle suspension, also may become less stable at elevated temperatures. Treatment fluids such as conformance gel compositions (e.g. chitosan-containing gels) degrade at temperatures as low as 275° F. within a day or two, completely lose their gel structure, and become thin fluids. Thus, these compositions may not be suitable for high temperature applications. This is particularly the case when such compositions are designed for long term performance, such as conformance gels.
A number of methods for adjusting the gelation time and/or improving thermal stability of crosslinkable-polymer compositions have been used heretofore. For instance, the gelation time of the above mentioned crosslinkable-polymer chitosan compositions may be lengthened by increasing the number of bulky and/or less-reactive monomers in the selected polymer or polymers used. Alternatively, the thermal stablility of the above mentioned crosslinkable-polymer chitosan compositions may be lengthened by the addition of oxygen scavengers to minimize oxygen mediated thermal degradation. These modifications, however, still may be inadequate to provide the desired gelation times for certain applications. Furthermore, these modifications do not stop thermal degradation completely and still may be inadequate to provide the desired thermal stability for certain applications.